Wait for the Best Software
If you want the best software experience HTC has to offer you'll probably want to hold out for the new U12.
The U12 or 12+ will launch with Android 8.0 Oreo on board. It'll also, according to recent rumors, come with some exciting new software from HTC. That's on top of Sense UI and a new 2nd generation "edge sense" squeezing technology.
Combine that with the Google Assistant, Android 8.0 Oreo, and lots of software enhancements and this could be a great alternative to the Galaxy S9.
This will be HTC's best version of Android Oreo and that alone might be compelling enough to convince you to wait a few weeks before buying a new device. But that's not all. You'll also want the latest phone for the 18 months or 2-years of software updates. So while you might be tempted to go with a cheaper device like the HTC U11 or Samsung Galaxy S8, these devices are already a year old which means they're already a year into their software support.
The end of life will come much faster for those phones. Combine all of that and HTC's simple yet elegant take on stock Android, and this phone could be worth the wait.